The invention relates to valves and particularly, though not exclusively, to gate valves.
A typical gate valve comprises a valve housing and an obturator movable in the housing between open and closed positions.
When the valve is closed or partly closed, the pressure is greater on one side of the obturator than on the other. This unbalanced pressure creates a force pushing the obturator against supporting surfaces in the housing.
When the valve is operated, this force creates a friction drag which resists movement of the valve member. The force required to operate the valve is proportional to the friction drag.
It is beneficial to reduce the operating force for both hand operated and power operated valves. The equipment for the power operation of valves is generally sold in a range of fixed sizes. If the operating force can be reduced sufficiently, it becomes possible to use a lower powered, lower cost operating device.